This invention generally relates to a color television (TV) signal converting circuit and, more particularly, to a PAL/NTSC color TV signal converting circuit that is preferably assembled in a video disc reproducing apparatus or a video tape recorder (VTR).
As is known in the art, the color TV signal of an NTSC TV system is composed of a luminance signal, as well as a carrier chrominance (referred to as a "chroma") signal, wherein two color difference signal components (R-Y and B-Y) of the carrier chrominance signal are quadrature-modulated on the sub-carrier.
When this NTSC TV signal is to be recorded on the recording medium, e.g., a video disc, the frequency of the carrier chroma signal is required to be converted into a lower frequency (e.g., 1.53 MHz) than the original (standard) one (e.g., 3.58 MHz), because restrictions exist in the available frequency band of the recording medium. Accordingly, when the picked up signals from the video disc are reproduced, a known reproducing system of the video disc recorded by the general NTSC system will be described with reference to FIG. 1.
The composite color TV signal which is detected from a video disc (not shown) and is to be reproduced is separated by a filter (not shown) into a chroma signal and a luminance signal, the latter of which has a synchronization signal (referred to as a "sync"). The chroma signal Ch and the luminance signal Y are applied to input terminals 11 and 12, respectively. The chroma signal Ch applied to the input terminal 11 is frequency-interleaved with the luminance signal EQU (Y signal) as follows:
195/2.multidot.fH=1.534091 (MHz),
where fH is the line frequency. To convert the 1.53 MHz chroma signal into the 3.58 MHz chroma signal before the 1.53 MHz chroma signal is converted to a lower frequency, the 1.53 MHz chroma signal is supplied to a frequency converter 13 and is multiplied with a 5.11 MHz (1.53+3.58) continuous wave signal (referred to as a "CW signal"). The 5.11 MHz CW signal is supplied from a voltage controlled oscillator 14. An output signal from the frequency converter 13 is supplied to a 3.58 MHz band-pass filter 15 which then produces a 3.58 MHz NTSC chroma signal. This 3.58 MHz chroma signal and the Y signal applied to the input terminal 12 are mixed by a mixer 16. An NTSC color TV signal then appears at an output terminal 17.
Meanwhile, a timing error of a signal component occurs in a video disc player due to the wow and flutter of a turntable. A small frequency fluctuation occurs in the 1.53 MHz carrier signal. An automatic phase control (APC) loop is therefore incorporated to eliminate such a fluctuation. The chroma signal the frequency of which is converted into 3.58 MHz is supplied to a phase comparator 18 and is then compared with a 3.58 MHz reference signal during a burst period. The 3.58 MHz reference signal is supplied from an oscillator 19. A sync signal is separated by a sync separator 20 from the Y signal applied to the input terminal 12 and is supplied to a burst gate pulse generator 21 which then produces a burst gate pulse at its output end. An output signal from the phase comparator 18 is held by a sample and hold circuit 22 for a 1 line period (1H period) and is then supplied to an oscillating frequency control terminal of the 5.11 MHz VCO 14 through a low-pass filter 23.
Meanwhile, a PAL color TV signal has, in addition to the Y-signal, two color difference signals which quadrature-modulate the sub-carrier in the same manner as the NTSC color TV signal. However, in the PAL color TV signal, the R-Y component is phase-inverted for every successive horizontal line. The carrier frequency fsc of the PAL signal is generally 4.43 MHz.
An example of a demodulation circuit for demodulating the PAL color TV signal is illustrated in FIG. 2. Referring to FIG. 2, a PAL composite TV signal is supplied to an input terminal 25 and is separated by a Ch-Y separator 26, so that the Y signal appears on a signal line 27 and a chroma signal appears on a signal line 28. This chroma signal is added by an adder 30 to a 1H-delayed chroma signal produced from a 1 line delay line 29. At the same time, the chroma signal appearing on the signal line 28 is subtracted by a subtractor 31 from the 1H-delayed chroma signal. As a result, the adder 30 produces a B-Y component and the subtractor 31 produces an R-Y component. The B-Y and R-Y components are supplied to a B-Y demodulator 32 and an R-Y demodulator 33, respectively. The signal line 28 is connected to a local sub-carrier generator 35 through a burst phase discriminator 34. A 4.43 MHz reference subcarrier is supplied from the local sub-carrier generator 35 to the B-Y demodulator 32 through a 90.degree. phase changing circuit 36 and, at the same time, is supplied to the R-Y demodulator 33 through a line changeover switch 37. The polarity of the R-Y component is inverted for every successive line, so that the phase of the reference sub-carrier applied to the R-Y demodulator 33 must be shifted by 180.degree. for every 1H. The line changeover switch 37 has a 180.degree. phase changing circuit 371 and a changeover switch 372. The switch 372 is then switched between a signal line 381 for a 180.degree. phase-shifted signal and a signal line 382 for a direct signal. The switching operation of the switch 372 is controlled by a line switching signal from a flip-flop 40 which receives the sync signal separated from the Y signal applied to the input terminal 25 by a sync separator 39. The sync detected signals from the B-Y and R-Y demodulators 32, 33 are supplied to a matrix circuit 41 which then produces a blue signal B, a green signal G and a red signal R.
However, when a PAL color TV receiver receives an NTSC signal, the NTSC luminance may be perceived as being of unnatural brightness, due to differences between standards used in the PAL and NTSC systems. For example, the number of scanning lines in the NTSC system is 525, whereas the number of scanning lines in the PAL system is 625. In the above-mentioned case, although the vertical width is contracted, marginal reproduction can be performed. However, the color signal processing systems of the PAL and NTSC chroma signals differ from each other, thus disabling reproduction.
A commercial demand is expected to arise for reproduction of an NTSC signal by a PAL color TV receiver in European countries where the PAL system is adopted. A commercial demand is also expected to arise for reproduction of a PAL signal by an NTSC color TV receiver. To satisfy these commercial demands, a method currently used in satellite broadcasting systems is proposed wherein an NTSC color TV signal is completely demodulated and the demodulated signal is converted to a PAL color TV signal so as to allow reproduction by the PAL color TV receiver, and vice versa.
However, in the case of reproducing an NTSC video disc by a PAL color TV receiver, the method used in the above-mentioned satellite broadcasting systems is difficult to apply to an electrical appliance such as a video disc player which is subject to restrictions as to, for example, its circuit scale and retail price.